The present invention relates to an optical distance measuring device for measuring a distance without contact and to a printing apparatus using the same.
FIG. 7 shows a configuration of an optical distance measuring device of a background art. In FIG. 7, light emitted by luminescence of a light emission element 1 is collimated by a collimating lens 2 and thereafter irradiates a measuring object 3. Reflected light from the measuring object 3 is condensed by a condensing lens 4 and is subsequently made incident on a photodetector 5. In this arrangement, a position of a spot of the light incident on the photodetector 5 varies according to a distance between the condensing lens 4 and the measuring object 3, as shown by dashed lines.
As the photodetector 5, therefore, a position sensing device (PSD) is used of which an output varies according to the position of the spot of the incident light. The distance from the optical distance measuring device to the measuring object 3 is thus detected by the position sensing device 5.
In an optical distance measuring device of a background art shown in FIG. 8, light emitted by luminescence of a light emission element 6 is condensed by a lens 7 so as to be slightly focused relative to parallel light and thereafter irradiates a measuring object 8. Reflected light from the measuring object 8 is condensed by a condensing lens 9 and is subsequently made incident on a photodetector 10. In this arrangement, a size (i.e., a spot diameter) of an image formed on the photodetector 10 varies according to a distance between the condensing lens 9 and the measuring object 8, as shown by dashed lines.
As the photodetector 10, therefore, an image sensor is used that has a light receiving surface divided into a number of regions in a two-dimensional manner. The spot size is detected by the image sensor 10 and the distance from the optical distance measuring device to the measuring object 8 is thereby detected.
In printing apparatus such as copying machines, printers, and facsimiles, such an optical distance measuring device as described above is used for detecting a thickness of a paper form without contact in order to improve printing quality.
The conventional optical distance measuring devices, however, have such problems as follows. That is, the conventional optical distance measuring devices use the method in which the measuring object 3, 8 is irradiated vertically with light from the light emission element 1, 6 and in which only light with specified angles of reflection out of light having undergone diffused reflection by the measuring object 3, 8 is condensed and is made incident on the photodetector 5, 10 (trigonometrical distance measuring method). Accordingly, the measuring object 3, 8 having a mirror surface or a close-to-mirror surface does not cause diffused reflection and thus makes it impossible to measure the distance thereto.
On condition that the distance between the measuring object 3, 8 and the optical distance measuring device is too small, besides, it may be impossible to measure the distance because light having undergone diffused reflection by the measuring object 3, 8 may not be advantageously incident on the photodetector 5, 10. Owing to use of light having undergone diffused reflection by the measuring object 3, 8, moreover, the position or diameter of the spot on the photodetector 5, 10 does not change greatly with change in the distance between the measuring object 3, 8 and the optical distance measuring device. It is therefore difficult to accurately detect a minute change in the distance.
It is an object of the present invention to provide an optical distance measuring device that is capable of measuring accurately a distance to a measuring object and a change in the distance even if a surface of the measuring object is a mirror surface or close to a mirror surface, even if the distance to the measuring object is small, or even if a change in the distance to the measuring object is minute, and to provide a printing apparatus using the same.